From the website:
on Windows, create a folder in C:/ProgramFiles/stencila and copy the stencila.exe file into it; then open Windows Command Line and add the folder to the PATH: setx PATH “$env:path;C:/ProgramFiles/stencila”

Should be:
on Windows, create a folder in C:/ProgramFiles/stencila and copy the stencila.exe file into it; then open Windows Command Line and add the folder to the PATH : setx PATH “$env:path;C:\ProgramFiles\stencila”